The frequency spectrum of a digital radio system is broken into channels that are small sub-spectrums. A first transmitter and receiver pair establishes a communication link over a first predetermined channel while other transmitter and receiver pairs use other predetermined channels. The transmitter transmits to the receiver over the channel using a predetermined data rate and modulation scheme (e.g., BPSK, QPSK, BFSK, QFSK).
Typically, a data transmission consists of three parts. The first part is an unmodulated carrier signal. The second part is a preamble of known information that is relatively easy for the receiver to detect and to synchronize with. The preamble may be, for example, a period of carrier signal modulated by a known training sequence (e.g., square wave) using a simple modulation scheme (i.e., BFSK). The third part of the data transmission is the modulated waveform that contains the unknown information data bits that are being transmitted.
The data rate of the transmission is usually measured in bits per second (bps), including kilobits per second (Kbps) and megabits per second (Mbps). The number of bits per second is related to the type of signaling (also known as encoding and modulation) that is used to convey the information and the number of times per second that the transmitted signal changes its value. For example, in a frequency-shift-keyed (FSK) digital signal radio system, data is encoded by generating frequency deviations away from the carrier frequency. Decoding the transmitted information entails measuring the frequency deviations away from the carrier frequency and inferring the transmitted information.
However, if the transmitted carrier frequency is at a frequency other than the nominal frequency the receiver expects, the measurement of frequency deviation becomes inaccurate. Thus, the performance and sensitivity of the receiver are degraded. This is a known problem in FSK digital radio systems. The above-described problem is depicted in greater detail in FIGS. 1A through 2B.
FIG. 1A illustrates a frequency-shift keyed (FSK) carrier signal that is properly aligned to a receiver reference carrier signal. The transmitted carrier frequency is shown as a solid line and the receiver carrier frequency is shown as a dotted line. When no data bits are being transmitted, the transmitter carrier signal is equal to some center frequency value, such as 600 MHz. The receiver carrier reference signal is aligned with the center frequency value. For, the sake of clarity, the dotted line representing the receiver carrier frequency is slightly offset in FIG. 1 from the solid line representing the transmitted carrier frequency so that the two lines do not coincide.
When data bits are transmitted, the frequency of the transmitted carrier signal is varied above and below the nominal or center frequency. These frequency variations are represented by the up and down arrows in FIG. 1A. For example, a Logic 1 may be transmitted by changing the transmitter frequency to 100 KHz above the center frequency and a Logic 0 may be transmitted by changing the transmitter frequency to 100 KHz below the center frequency. Thus, in the exemplary embodiment, a Logic 1 would be transmitted as 600.1 MHz and a Logic 0 would be transmitted as 599.9 MHz.
Within the receiver, the frequency variations in the transmitted carrier signal are translated into amplitude variations in the output voltage of a frequency discriminator or a similar circuit. FIG. 1B illustrates the amplitude modulated output of a frequency discriminator receiving an FSK carrier signal that is properly aligned with a reference voltage representing the receiver reference carrier signal. The amplitude modulated output voltage of the frequency discriminator is shown as a solid line and the reference voltage representing the receiver carrier frequency is shown as a dotted line. For the sake of clarity, the dotted line representing the amplitude modulated output voltage is slightly offset in FIG. 1B from the solid line representing the reference voltage so that the two lines do not coincide.
The amplitude modulated output voltage of the frequency discriminator is compared to the reference voltage to determine the value of the transmitted data. When no data bits are being transmitted, the amplitude modulated output voltage is equal to the reference voltage. When a Logic 1 data bit is transmitted and the transmitter frequency increases to, for example, 100 KHz above the center frequency, the frequency discriminator increases the amplitude modulated output voltage above the reference voltage. When a Logic 0 data bit is transmitted and the transmitter frequency decreases to, for example, 100 KHz below the center frequency, the frequency discriminator decreases the amplitude modulated output voltage below the reference voltage. A voltage comparator circuit translates the voltage differences into Logic 1 values and Logic 0 values. In the example shown in FIGS. 1A and 1B, the data sequence 101100 has been transmitted.
FIG. 2A illustrates a frequency-shift keyed (FSK) carrier signal that is not properly aligned to the receiver reference carrier signal. The transmitted carrier frequency has drifted to a higher center frequency than in FIGS. 1A and 1B. The transmitted carrier frequency is shown as a solid line and the receiver carrier frequency is shown as a dotted line. The receiver carrier reference frequency is so far below the new transmitted carrier frequency that positive and negative frequency variations of the transmitted carrier signal above and below the new center frequency are both higher than the receiver carrier frequency. Thus, positive and negative frequency variations are both represented by up arrows in FIG. 2A.
FIG. 2B illustrates the amplitude modulated output of a frequency discriminator receiving an FSK carrier signal that is misaligned with a reference voltage representing the receiver reference carrier signal. The amplitude modulated output voltage of the frequency discriminator is shown as a solid line and the reference voltage representing the receiver carrier frequency is shown as a dotted line. As a result of the increase in the transmitted carrier frequency, the receiver reference voltage is so far below the amplitude modulated output voltage of the frequency discriminator that positive and negative amplitude variations in the amplitude modulated output voltage are both higher than the reference voltage. As a result, comparison of the amplitude modulated output voltage and the reference voltage translates the voltage differences into inaccurate Logic 1 and Logic 0 values. In the example shown in FIGS. 2A and 2B, the transmitted data sequence is inaccurately determined to be 111111.
A receiver most accurately decodes the message when the receiver evaluates (or estimates) a bit level at the center of the bit (symbol) interval. Further degradation of the receiver performance occurs if the receiver measurements are not actually aligned to the bit (or symbol) center. During acquisition, the receiver and transmitter are synchronized in time and frequency by the preamble. However, once the receiver starts decoding the information, the receiver and transmitter drift apart in both time and frequency.
One method to re-synchronize the transmitter and the receiver frequencies is to measure the frequency difference and correct for it in the receiver. To synchronize the timing between the transmitter and receiver, the receiver must look for a known information pattern in the signal and align the decision timing in the receiver to optimize the decoding of the known information pattern. Many methods have been proposed and implemented to accomplish this.
As noted above, a conventional transmission consists of three parts: 1) an unmodulated signal, 2) a known preamble, and 3) a message containing unknown information. Typically, during signal acquisition, the receiver uses the information in the unmodulated signal and in the known preamble to attain accurate synchronization. During the third part of the message, the receiver may use a sequence-estimator demodulator to improve demodulation of the unknown information. Using this information (a decision-directed approach), the receiver tracks the time and frequency differences between the receiver and transmitter and corrects for the differences.
Traditionally, early-late gate symbol synchronization has been used in many communication systems. The operation of an early-late gate is based on the fact that the matched filtered demodulation produces an auto-correlation function that is symmetric and peaks at the optimum sampling time. FIG. 3 illustrates the relationship between the demodulated output of a frequency discriminator receiving a FSK signal and the auto-correlation function that the receiver produces to perform alignment with the received FSK signal. The demodulated waveform is a sequence of square-wave pulses and the auto-correlation function is a sequence of sawtooth-like signal peaks. At sampling time T1, a positive-going peak in the auto-correlation function is correctly aligned with the center of a Logic 1 bit in the demodulated waveform. At sampling time T2, a negative-going peak in the auto-correlation function is correctly aligned with the center of a Logic 0 bit in the demodulated waveform. If the peaks in the auto-correlation function drift away from the center of the bit periods, the early-late gate detection circuitry of the receiver automatically adjusts timing and frequency of the receiver reference signals to track and correct for the drift.
However, conventional demodulators typically make hard decisions on information symbols based on a single threshold value. This results in sub-optimal performance in the decision-making block. In particular, conventional sequence-estimation demodulators often make mistakes on bit levels that are different than the preceding and trailing bit levels, such as a 101 sequence or a 010 sequence.
Therefore, there is a need in the art for improved frequency shift keyed (FSK) receivers that are capable of more accurately adjusting for frequency and timing drifts between the incoming transmitted carrier frequency and the receiver carrier reference signal. In particular, there is a need for FSK receivers that are capable of more accurately determining bit levels that are different than the preceding and trailing bit levels.